Unwanted sounds, such as loud sounds or sounds that have a rapid relative increase in level, may be produced by telephone or radio systems, intentionally or unintentionally. Those sounds are typically experienced by a user through headphones or a headset.
Without protection against those sounds, the user may experience a phenomenon known as acoustic shock. Acoustic shock may result in permanent hearing loss, temporary hearing loss and tinnitus (constant ringing in the ears). Sufferers also report symptoms including extreme pain, vertigo and burning sensations. One of the main issues in the cause of acoustic shock syndrome, apart from the initial high-level sound, is the startle reflex action. This reaction can cause numerous muscles to activate to an unusual degree.
In order to prevent the user from experiencing acoustic shock, the following approaches are provided in telephone systems: 1—Automatic gain control; 2—Adjustable notch filters to remove narrow band tones or “shrieks” when they are detected; 3—Clipping of high level signals using diodes or similar devices:
These approaches have also been combined. All of these approaches use techniques that are well known in the art and have been seen in other application areas (e.g. hearing aids).
The existing devices offer some protection. However, the processed output signal of those devices has reduced fidelity compared to an input signal. Typical distortions of the signal include: “pumping” (unnecessary and audible adjustments of the gain that adversely affect the perceived quality of the processed speech signal) and “holes” (audio dropouts in the processed signal caused by extreme gain adjustments), harmonic distortion as well as the accompanying intermodulation distortion that comes from poor gain control. More complex systems may also suffer from excessive input-output latency (i.e., group delay), which can adversely impact network, acoustic and line echo cancellers.
Currently there are specifications under development that provide guidelines and recommendations for the performance of acoustic shock systems. These include: ITU-T Recommendation P.360 “Efficiency of devices from preventing the occurrence of acoustic pressure by telephone receivers”; UK standard BS6317 specified continuous signals only; US standard, UL1950; EN60950 (http://www.ktl.com/telecoms/shock.htm).
To meet these emerging performance requirements and deliver high-fidelity with low group delay, a new approach is needed. It is, therefore, desirable to provide a new method and system that can fully protect a user against the acoustic shock so as to meet the above guidelines and future guidelines that may emerge.